1. Field of the Invention
The present invention relates to a radio frequency identification (RFID) communication system and, more particularly, to a radio frequency identification (RFID) communication system and the search method thereof.
2. Description of Related Art
FIG. 1 is a schematic diagram of essential devices of a typical RFID communication system. In FIG. 1, the system includes a reader 11 and a transponder 12. As shown in FIG. 1, the operation principle for the system is that the reader 11 sends a carrier with instructions to the transponder 12 and the transponder 12 obtains DC power via rectifying the carrier. Further, a demodulator inside the transponder demodulates instructions on the carrier, thereby responding the reader or sending required data according to the instructions demodulated.
RFID communication system is increasing quickly and its applications become more diversified, thus typical one-to-one communication cannot meet existing applications. For example, a reader may communicate with a plurality of transponder. FIG. 2 is a schematic diagram of one reader 21 to multiple transponders 22–25. When the transponders 22–25 concurrently send responds to the reader 21, signal interfere among the transponders can cause collision.
Currently, three solutions are applied to prevent collisions, which, as shown in FIG. 2, are spatial domain, frequency domain and time domain. For spatial domain, it is assumed that the transponders 22–25 are separated by a space from each other. Such a solution is applied mostly to microwave systems and can discriminate each object by means of directional antenna. However, such a solution is hard in design for a non-microwave system. For frequency domain, the reader 21 sends a carrier and instructions to the transponder 22–25 at fixed frequency points (band) and the transponders 22–25 respectively selects, according to instructions decoded, one from corresponding multiple back-transmittable frequencies in order to send corresponding ID codes to the reader 21. Since the transponders 22–25 send the ID codes back to the reader 21 in different frequencies, the collision is avoided. However, such a solution costs very high and is limited in specific applications. For time domain, the transponders 22–25 are scheduled such that each of the transponders 22–25 can send its own data in the scheduled time.
One more typical collision solution mostly seen is using polling for searching. Namely, one-to-one roll call is applied for searching. However, such a solution has a poor performance when the number of transponders is large. Accordingly, current collision solution generally adopts binary search algorithm in time domain to thus quickly obtain ID codes of all transponders, or random number method, i.e., using the transponders to generate random numbers to accordingly determine each transponder's transmission time. Since each of the transponders generates different random numbers and thus has different response time, signal interfere probability among the transponders is relatively reduced when random space is much greater than transponder number, such that ID codes of the transponders in read range can be read. Generally, upon read efficiency increase in the random number method, a number of read instructions are increased. For example, a mute instruction is applied to make transponders accurately read corresponding ID codes enter in a mute mode, thereby reducing transponder number facing a reader.
Operation principles respectively for the cited binary search algorithm and the random number method are described as follows. The binary search algorithm is shown in FIGS. 3 and 4. FIG. 3 is a schematic diagram of the binary search algorithm. FIG. 4 is a flowchart of communication between a read 41 and a transponder 42. As shown in FIGS. 3 and 4, for searching ID codes of 0000 and 0011 in a given example of four bits, a search is performed sequentially from MSB to LSB and a collision occurs at third bits of 0000 and 0011. When the reader 41 sees the collision, the reader 41 sets searching 000 firstly until 0000 ID code is found, and then 001 until 0011 ID code is found. As such, applying the binary search algorithm is simple and quick but heavy communication between the reader 41 and the transponder 42 is required, which needs guard time for switch between the reader 41 and the transponder 42 in order to avoid error caused by the switch. However, it wastes time and reduces entire performance.
FIG. 5 is a timing of every transponder using the random number method. As shown in FIG. 5, every transponder generates a random number and a reader sets a response cycle. Next, the random numbers determine corresponding periods for transponders respectively. For example, the response cycle is 4, i.e., transmission every 4 periods, and every transponder has different duration for one period, determined by the random number. Accordingly, transmission time for every transponder is different such that every ID code in response can be read accurately. As shown in FIG. 5, ID code of a transponder C is read first, then ID code of a transponder A is read and final ID code of a transponder B is read. In some random number methods, a random number is compared with a value preset by a reader, and an ID code corresponding to the random number can be transmitted when the random number has the same value as the reader or conversely the ID code cannot be transmitted. However, applying the random number method may cause no response signal during a certain time, and both occurrence point and duration regarding the certain time are unpredictable, thus leading to poor time efficiency. For example, time is wasted at Tc1–Tc4 and Ta2–Ta4 of FIG. 5.
Therefore, it is desirable to provide an improved system and method to mitigate and/or obviate the aforementioned problems.